New Lasers Fight Crime, Martians

A new technique that uses a laser to vaporize materials like rocks and steel to analyze their chemical composition is finding new applications from Mars to forensics.

Thanks to its relatively small size and low cost, laser-induced breakdown spectroscopy is emerging from the laboratory and turning into a precise tool for figuring out what something is made of. What had been a technique largely for scientists now can be transformed into a tough, small system that can be operated by a technician instead of a PhD.

“The same things that make it amenable to go to Mars also make it amenable to go out in the field,” said Jose Almirall, a chemist at Florida International University who has a grant from the Department of Justice to explore how crime labs can use the technology.

NASA will be deploying a LIBS system called the “ChemCam” on its new Mars rover, now named Curiosity and scheduled to launch next year.

LIBS works by blasting a material with a high-energy laser pulses. The Mars Curiosity rover will send an average of three pulses a second, each one 5 nanoseconds long. The power during that pulse is in the range of 10 megawatts.

That’s not enough to shoot a hole in your hand, but it’ll leave a mark.

“I’ve shot myself and you might see a little spot where you shot yourself, if it’s just one laser shot,” said Roger Wiens, a space scientist at Los Alamos National Laboratory, who developed the ChemCam for the Mars Curiosity mission.

In some cases, the ChemCam will be focused on rocks for more than 15 seconds. If you were to do that to your arm, “You’d be in pain,” Wiens said.

There are no plans to use the laser for anything but science, but there’s something science-fictionally satisfying about the next Mars rover coming equipped with a laser “gun.”

The laser shots vaporize a crater less than a millimeter across, turning its molecules into a 14,000-degree plasma. The atoms are shorn of their electrons, but as the plasma ball cools down, they return to a more normal state. The electrons drop into their orbits around the nucleus and as they do so, the little plasma ball emits light.

“You can see it with your eyes and it makes a little zapping sound when you do it,” said Wiens.

The specific color of the light tells scientists exactly what element they are looking at if they pass it through a spectrometer, which can precisely measure the wavelength of light.

Lasers have been used to create small clouds of atoms for spectral analysis in the past, but those systems require a separate torch that heats the atoms into a plasma. With LIBS, the same laser blast does both jobs. It’s this simplicity that could aid the technology’s spread. A 2006 Cambridge University Press book on LIBS declared the technique “perhaps the most versatile method yet developed for elemental analysis.”

On Earth, there are plenty of times when one might want to know the very precise composition of a substance. In late 2009, for the first time, a LIBS analysis was used in a court. It helped identify a would-be bank robber by the glass on his clothing in Maryland. After being locked in a vestibule by secret alarms, he shot his way out and sped away. After being apprehended, some pieces of glass were found on his clothes. When Almirall’s team compared the precise composition of that glass with the stuff from the bank, they came back with a probable match.

Glass made in different places and at different times is quite distinctive at the molecular level, Almirall said. In modern manufacturing processes, sodium carbonate and calcium oxide are usually added to the base silica. But tiny amounts of other elements such as strontium can act as a particular type of glass’ signature. The older a glass-manufacturing plant is, for example, the more zirconium leaches into the glass melt.
“The glass manufacturers don’t care if they have 10 or 20 or 30 parts per million of strontium,” said Almirall. “We’ll look at seven, eight, nine different elements in the raw material to detect differences.”

The level of precision that LIBS can offer may eventually help manufacturers do quality control on their products. Almirall is even thinking about how to use a briefcase LIBS system to test products for U.S. Customs to ensure their safety and provenance.

“If you were receiving some toys and want to immediately know if you have lead in these toys, LIBS could do that very quickly,” Almirall said. “Green light, no lead. Red light, there is lead.”

Several companies are trying to commercialize LIBS systems including Applied Spectra of Fremont, California, and Stellar Net of Tampa, Florida. Stellar Net’s PortaLibs is shown above. Clearly, the technology has gone a long way toward fulfilling the promise Wiens first saw in it 13 years ago.

“I was looking for other technologies to put into space and a colleague here [at Los Alamos Lab] took me to his laboratory and showed me a little laser the size of a cigar and a rock across the room and a little transistor battery,” Wiens recalled. “He had the laser hooked up to the 9-volt battery, charged up some capacitors for a few seconds, and then zap, across the room, there was a spark.”

Update 2/18: This story has been corrected to show the correct publication date for the book, LASER-INDUCED BREAKDOWN SPECTROSCOPY: Fundamentals and Applications.

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